A simple, sensitive and rapid liquid chromatography/electrospray ionization tandem mass spectrometry (LC–ESI-MS/MS) method was developed and validated for simultaneous quantification of naloxone, buprenorphine and its metabolite norbuprenorphine in human plasma. Human plasma samples were extracted using a single step liquid–liquid extraction, and then separated on an Imtakt Unison UK-C18 column (2.1 × 50 mm, 3 μm) using alkaline mobile phases with gradient elution. All of the analytes were detected in positive ion mode using multiple reaction monitoring (MRM). The method was validated and the specificity, linearity, lower limit of quantitation, precision, accuracy, recoveries and stability were determined. The linear range was 20–10000 pg/mL for buprenorphine and norbuprenorphine; and 1–500 pg/mL for naloxone. The correlation coefficient (R2) values for all three analytes were ≥0.995. The precision and accuracy for intra-day and inter-day were <11.0%. The recoveries were >63% and matrix effects were tracked by the deuterated internal standards (IS) with the IS-normalized matrix factor ranging from 0.96 to 1.33 for all three analytes. The validated method was successfully applied in a clinical pharmacokinetic study with low dose administration of sublingual buprenorphine and naloxone.